Superelastic guiding member

ABSTRACT

An improved guiding member for use within a body lumen having a unique combination of superelastic characteristics. The superelastic alloy material has a composition consisting of about 30% to about 52% (atomic) titanium, and about 38% to 52% nickel and may have one or more elements selected from the group consisting of iron, cobalt, platinum, palladium, vanadium, copper, zirconium, hafnium and niobium. The alloy material is subjected to thermomechanical processing which includes a final cold working of about 10 to about 75% and then a heat treatment at a temperature between about 450° and about 600° C. and preferably about 475° to about 550° C. Before the heat treatment the cold worked alloy material is preferably subjected to mechanical straightening. The alloy material is preferably subjected to stresses equal to about 5 to about 50% of the room temperature ultimate yield stress of the material during the thermal treatment. The guiding member using such improved material exhibits a stress-induced austenite-to-martensite phase transformation at an exceptionally high constant yield strength of over 90 ksi for solid members and over 70 ksi for tubular members with a broad recoverable strain of at least about 4% during the phase transformation. An essentially whip free product is obtained.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/212,431,filed Mar. 11, 1994, now abandoned, which is a continuation-in-part ofapplication Ser. No. 07/994,679, filed Dec. 22, 1992, now U.S. Pat. No.5,341,818, which is a continuation-in-part of application Ser. No.07/629,381, filed Dec. 18, 1990, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the field of medical devices, and moreparticularly to guiding means such as guidewires for advancing catheterswithin body lumens in procedures such as percutaneous transluminalcoronary angioplasty (PTCA).

In typical PTCA procedures a guiding catheter having a preformed distaltip is percutaneously introduced into the cardiovascular system of apatient in a conventional Seldiger technique and advanced therein untilthe distal tip of the guiding catheter is seated in the ostium of adesired coronary artery. A guidewire is positioned within an inner lumenof a dilatation catheter and then both are advanced through the guidingcatheter to the distal end thereof. The guidewire is first advanced outof the distal end of the guiding catheter into the patient's coronaryvasculature until the distal end of the guidewire crosses a lesion to bedilated, then the dilatation catheter having an inflatable balloon onthe distal portion thereof is advanced into the patient's coronaryanatomy over the previously introduced guidewire until the balloon ofthe dilatation catheter is properly positioned across the lesion. Oncein position across the lesion, the balloon is inflated to apredetermined size with radiopaque liquid at relatively high pressures(e.g. greater than 4 atmospheres) to compress the arterioscleroticplaque of the lesion against the inside of the artery wall and tootherwise expand the inner lumen of the artery. The balloon is thendeflated so that blood flow is resumed through the dilated artery andthe dilatation catheter can be removed therefrom.

Conventional guidewires for angioplasty and other vascular proceduresusually comprise an elongated core member with one or more taperedsections near the distal end thereof and a flexible body such as ahelical coil disposed about the distal portion of the core member. Ashapable member, which may be the distal extremity of the core member ora separate shaping ribbon which is secured to the distal extremity ofthe core member extends through the flexible body and is secured to arounded plug at the distal end of the flexible body. Torquing means areprovided on the proximal end of the core member to rotate, and therebysteer, the guidewire while it is being advanced through a patient'svascular system.

Further details of dilatation catheters, guidewires, and devicesassociated therewith for angioplasty procedures can be found in U.S.Pat. No. 4,323,071 (Simpson et al.); U.S. Pat. No. 4,439,185(Lundquist); U.S. Pat. No. 4,516,972 (Samson); U.S. Pat. No. 4,538,622(Samson et al.); U.S. Pat. No. 4,554,929 (Samson et al.); U.S. Pat. No.4,616,652 (Simpson); and U.S. Pat. No. 4,638,805 (Powell).

Steerable dilatation catheters with fixed, built-in guiding members,such as described in U.S. Pat. No. 4,582,181 (now Re 33,166) arefrequently used because they have lower deflated profiles thanconventional over-the-wire dilatation catheters and a lower profileallows the catheter to cross tighter lesions and to be advanced muchdeeper into a patient's coronary anatomy.

A major requirement for guidewires and other guiding members, whetherthey be solid wire or tubular members, is that they have sufficientcolumnar strength to be pushed through a patient's vascular system orother body lumen without kinking. However, they must also be flexibleenough to avoid damaging the blood vessel or other body lumen throughwhich they are advanced. Efforts have been made to improve both thestrength and flexibility of guidewires to make them more suitable fortheir intended uses, but these two properties are for the most partdiametrically opposed to one another in that an increase in one usuallyinvolves a decrease in the other.

The prior art makes reference to the use of alloys such as NITINOL,which is an acronym for Ni-Ti Naval Ordnance Laboratory. These alloyshave shape memory and/or superelastic characteristics and may be used inmedical devices which are designed to be inserted into a patient's body.The shape memory characteristics allow the devices to be deformed tofacilitate their insertion into a body lumen or cavity and then beheated within the body so that the device returns to its original shape.Superelastic characteristics on the other hand generally allow the metalto be deformed and restrained in the deformed condition to facilitatethe insertion of the medical device containing the metal into apatient's body, with such deformation causing the phase transformation.Once within the body lumen the restraint on the superelastic member canbe removed, thereby reducing the stress therein so that the superelasticmember can return to its original undeformed shape by the transformationback to the original phase.

Alloys having shape memory/superelastic characteristics generally haveat least two phases. These phases are a martensite phase, which has arelatively low tensile strength and which is stable at relatively lowtemperatures, and an austenite phase, which has a relatively hightensile strength and which is stable at temperatures higher than themartensite phase.

Shape memory characteristics are imparted to the alloy by heating themetal at a temperature above which the transformation from themartensite phase to the austenite phase is complete, i.e. a temperatureabove which the austenite phase is stable. The shape of the metal duringthis heat treatment is the shape "remembered". The heat treated metal iscooled to a temperature at which the martensite phase is stable, causingthe austenite phase to transform to the martensite phase. The metal inthe martensite phase is then plastically deformed, e.g. to facilitatethe entry thereof into a patient's body. Subsequent heating of thedeformed martensite phase to a temperature above the martensite toaustenite transformation temperature causes the deformed martensitephase to transform to the austenite phase and during this phasetransformation the metal reverts back to its original shape.

The prior methods of using the shape memory characteristics of thesealloys in medical devices intended to be placed within a patient's bodypresented operational difficulties. For example, with shape memoryalloys having a stable martensite temperature below body temperature, itwas frequently difficult to maintain the temperature of the medicaldevice containing such an alloy sufficiently below body temperature toprevent the transformation of the martensite phase to the austenitephase when the device was being inserted into a patient's body. Withintravascular devices formed of shape memory alloys havingmartensite-to-austenite transformation temperatures well above bodytemperature, the devices could be introduced into a patient's body withlittle or no problem, but they had to be heated to themartensite-to-austenite transformation temperature which was frequentlyhigh enough to cause tissue damage and very high levels of pain.

When stress is applied to a specimen of a metal such as NITINOLexhibiting superelastic characteristics at a temperature above which theaustenite is stable (i.e. the temperature at which the transformation ofmartensite phase to the austenite phase is complete), the specimendeforms elastically until it reaches a particular stress level where thealloy then undergoes a stress-induced phase transformation from theaustenite phase to the martensite phase. As the phase transformationproceeds, the alloy undergoes significant increases in strain but withlittle or no corresponding increases in stress. The strain increaseswhile the stress remains essentially constant until the transformationof the austenite phase to the martensite phase is complete. Thereafter,further increase in stress are necessary to cause further deformation.The martensitic metal first yields elastically upon the application ofadditional stress and then plastically with permanent residualdeformation.

If the load on the specimen is removed before any permanent deformationhas occurred, the martensitic specimen will elastically recover andtransform back to the austenite phase. The reduction in stress firstcauses a decrease in strain. As stress reduction reaches the level atwhich the martensite phase transforms back into the austenite phase, thestress level in the specimen will remain essentially constant (butsubstantially less than the constant stress level at which the austenitetransforms to the martensite) until the transformation back to theaustenite phase is complete, i.e. there is significant recovery instrain with only negligible corresponding stress reduction. After thetransformation back to austenite is complete, further stress reductionresults in elastic strain reduction. This ability to incur significantstrain at relatively constant stress upon the application of a load andto recover from the deformation upon the removal of the load is commonlyreferred to as superelasticity or pseudoelasticity.

The prior art makes reference to the use of metal alloys havingsuperelastic characteristics in medical devices which are intended to beinserted or otherwise used within a patient's body. See for example,U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445 (Sakamotoet al.).

The Sakamoto et al. patent discloses the use of a nickel-titaniumsuperelastic alloy in an intravascular guidewire which could beprocessed to develop relatively high yield strength levels. However, atthe relatively high yield stress levels which cause theaustenite-to-martensite phase transformation characteristic of thematerial, it did not have a very extensive stress-induced strain rangein which the austenite transforms to martensite at relative constantstress. As a result, frequently as the guidewire was being advancedthrough a patient's tortuous vascular system, it would be stressedbeyond the superelastic region, i.e. develop a permanent set or evenkink which can result in tissue damage. This permanent deformation wouldgenerally require the removal of the guidewire and the replacementthereof with another.

Products of the Jervis patent on the other hand had extensive strainranges, i.e. 2 to 8% strain, but the relatively constant stress level atwhich the austenite transformed to martensite was very low, e.g. 50 ksi.

What has been needed and heretofore unavailable is an elongated solid ortubular body for intravascular devices, such as guide wires or guidingmembers, which have at least a portion thereof exhibiting superelasticcharacteristics including an extended strain region over a relativelyconstant high stress level which effects the austenite transformation tomartensite and still provide a one-to-one torque response. The presentinvention satisfies these and other needs.

SUMMARY OF THE INVENTION

The present invention is directed to an improved superelastic body whichis suitable for intraluminal devices, such as guidewires or guidingmembers, wherein superelastic characteristics result from thestress-induced transformation of austenite to martensite.

The superelastic alloy body of the invention has a stable austenitephase which will transform to martensite phase upon the application ofstress and will exhibit a recoverable strain of at least about 4% uponthe stress induced transformation of the austenite phase to themartensite phase. The formation of the alloy body includes a final coldworking about 10 to about 75% and then a final memory imparting heattreatment at a temperature of about 450° to about 600° C., preferably475° to about 550° C. The cold worked, heat treated product exhibits astress-induced phase transformation at temperatures below about 45° C.at a relatively high stress level, e.g. above about 70 ksi, (483 Mpa)preferably above about 90 ksi (620 Mpa) for solid products and about 40ksi (276 Mpa) for hollow tubular products. The superelastic productexhibits at recoverable strain of at least 4% upon completion of thestress-induced transformation of the austenite phase to the martensitephase. The onset of the stress induced phase change from austenite tomartensite, preferably begins when the specimen has been strained about2% and extends to a strain level of about 8% at the completion of thephase change. The stress and strain referred to herein is measured bytensile testing. The stress-strain relationship determined by applying abending moment to a cantilevered specimen is slightly different from therelationship determined by tensile testing because the stresses whichoccur in the specimen during bending are not as uniform as they are intensile testing. The stress change during the phase transformation ismuch less than the stress either before or after the stress-inducedtransformation. In some instances the stress level during the phasechange is almost constant.

The elongated portion of the guiding member having superelasticproperties is preferably formed from an alloy consisting essentially ofabout 30 to about 52% titanium, about 38% to about 52% nickel andadditional alloying elements in amount up to 20% for copper and up toabout 10% in the case of other alloying elements. The additionalalloying elements may be selected from the group consisting of up to 3%each of iron, cobalt, chromium, platinum, palladium, zirconium, hafniumand niobium and up to about 10% vanadium. At nickel levels above 52% thealloy becomes too brittle to fabricate by cold working. Metallurgicallythe alloy consist essentially of a predominant quantity of a NiTiintermetallic compound and small quantities of other constituents.Additionally, when nickel is in excess Ni₃ Ti is formed, whereas whentitanium is in excess Ti₂ Ni is formed. As used herein, all referencesto percent alloy compositions are atomic percent unless otherwise noted.

To form the elongated superelastic portion of the guiding member,elongated solid rod or tubular stock of the preferred alloy material isfirst thermomechanically processed through a series of cold working,e.g. drawing and inter-annealing at temperatures between about 600° toabout 800° C. for about 5 to about 30 minutes and is then given afinal-cold working e.g. drawing, to effect a final size reduction ofabout 10% up to about 75% in the transverse cross section thereof. Forsolid products the final cold work is preferably about 30 to about 70%and for hollow tubular products the final cold work is preferably about10% to about 40%. As used herein % cold work is the size reduction ofthe transverse dimension of the work piece effected by the cold working.After the final cold working, the material is given a heat treatment ata temperature of about 450° to about 600° C., preferably about 475° toabout 550° C., for about 0.5 to about 60 minutes to generate thesuperelastic properties. To impart a straight memory, the cold workedmaterial may be subjected to a longitudinal stress equal to about 5% toabout 50%, preferably about 10% to about 30%, of the yield stress of thematerial (as measured at room temperature) during a heat treatment ofabout 450° to about 600° C. This thermomechanical processing imparts astraight "memory" to the superelastic portion and provides a relativelyuniform residual stress in the material. Preferably, the final coldworked product is subjected to mechanically straightening between thefinal cold working and heat treating steps to provide the wire ortubular product with substantially improved, one-to-one torque response,i.e. it is substantially whip free. The alloy composition and thermaltreatment are selected to provide an austenite finish transformationtemperature generally about -20° to about 40° C. and usually less thanbody temperature (approx. 37° C.). To obtain more consistent finalproperties, it is preferred to fully anneal the solid rod or tubularstock prior to cold working so that the material will always have thesame metallurgical structure at the start of the cold working. Thepre-annealing also ensures adequate ductility for cold working. It willbe appreciated by those skilled in the art that the alloy can be coldworked in a variety of ways other than drawing, such as rolling orswaging. The constant stress levels for tubular products have been foundto be slightly lower than the constant stress levels for solid productsdue to the inability to cold work the tubular products to the extent thesolid products can be cold worked. For example, solid superelastic wirematerial of the invention can have a relatively constant stress levelabove about 70 ksi, usually above about 90 ksi, whereas, hollowsuperelastic tubing material of the invention can have a relativelyconstant stress level above about 50 ksi (345 Mpa) usually above about60 ksi (414 Mpa). The ultimate tensile strength of both forms of thematerial is well above 200 ksi (1380 Mpa) with an ultimate elongation atfailure of about 15%.

The elongated body of the invention exhibits a stress-inducedaustenite-to-martensite phase transformation over a broad region ofstrain at very high, relatively constant stress levels. As a result aguiding member formed of this material is very flexible, it can beadvanced through very tortuous passageways such as a patient's coronaryvasculature with little risk that the superelastic portion of theguiding member will develop a permanent set and at the same time it willeffectively transmit the torque applied thereto without causing theguiding member to whip.

These and other advantages of the invention will become more apparentfrom the following detailed description thereof when taken inconjunction with the following exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a guidewire which embodies features of the invention.

FIG. 2 illustrates another embodiment of a guidewire of the invention.

FIG. 3 is a partial side elevational view, partially in section, of aguiding member embodying features of the invention which is incorporatedinto a fixed-wire dilatation catheter adapted for balloon angioplastyprocedures.

FIG. 4 is a schematic, graphical illustration of the stress-strainrelationship of superelastic material.

FIG. 5 illustrates another embodiment of a guidewire embodying featuresof the invention.

FIG. 6 illustrates the effects of the temperature during heat treatmentafter final cold working on the final austenite transformationtemperature (Af).

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a guidewire 10 embodying features of the inventionthat is adapted to be inserted into a body lumen such as an artery. Theguidewire 10 comprises an elongated body or core member 11 having anelongated proximal portion 12 and a distal portion 13, at least part ofwhich, preferably the distal portion, is formed of superelastic materialof the invention. The distal portion 13 has a plurality of sections 14,15 and 16 having sequentially smaller diameters with tapered sections17, 18 and 19 connecting the smaller diameter sections with adjacentsections. The elongated proximal portion 12 is provided with a femaledistal end 20 which receives the male end 21 of the distal portion 13.The ends 20 and 21 may be press fit together or may be secured togetherby means such as a suitable adhesive or by welding, brazing orsoldering.

A helical coil 22 is disposed about the distal portion 13 and has arounded plug 23 on the distal end thereof where the distal end of thehelical coil is welded to the distal end of a shaping ribbon 24 which issecured by its proximal end to the distal portion 13 by suitable means(e.g. brazing) at location 25. The coil 22 is also secured to the distalportion 13 of the elongated body 11 at location 25 and to the taperedsection 17 at location 26. Preferably, the most distal section 27 of thehelical coil 22 is made of radiopaque metal such as platinum or alloysthereof to facilitate the fluoroscopic observation of the distal portionof the guidewire while it is disposed within a patient's body.

The exposed portion of the elongated body 11 should be provided with acoating 28 of lubricous material such as polytetrafluoroethylene (soldunder the trademark Teflon by du Pont) or other suitable lubricouscoatings such as the polysiloxane coatings disclosed in co-pendingapplication Ser. No. 559,373, filed Jul. 24, 1990 which is herebyincorporated by reference.

FIG. 2 illustrates another embodiment of a guidewire which incorporatesfeatures of the invention. This embodiment is very similar to theembodiment shown in FIG. 1 except that the entire elongated body 11 isformed of material having superelastic characteristics and the distalportion 13 of the core member 11 extends all the way to the plug 23 andis preferably flattened at its most distal extremity 29 as ribbon 24 inthe embodiment shown in FIG. 1. All of the parts of the guidewire shownin FIG. 2 which correspond to the parts shown in FIG. 1 are numbered thesame as in FIG. 1.

FIG. 3 illustrates a fixed wire, steerable dilatation catheter 40 whichhas incorporated therein a guiding member 41 in accordance with theinvention. In this embodiment, the catheter 40 includes an elongatedtubular member 42 having an inner lumen 43 extending therein and aninflatable, relatively inelastic dilatation balloon 44 on the distalextremity of the tubular member. Guiding member 41 which includes anelongated body 45, a helical coil 46 disposed about and secured to thedistal end of the elongated body 45 and a shaping ribbon 50 extendingfrom the distal end of the elongated body to rounded plug 51 at thedistal end of the coil 46. The proximal portion 52 of the elongated body45 is disposed within the inner lumen 43 of the tubular member 42 andthe distal portion 53 of the elongated body 45 extends through theinterior of the dilatation balloon 44 and out the distal end thereof.The distal end of the balloon 44 is twisted and sealed about the distalportion 53 of the elongated body 45 extending therethrough in a mannerwhich is described in more detail in copending application Ser. No.521,103, filed May 9, 1990, which is hereby incorporated by reference.The helical coil 46 is secured to the distal portion 53 of the elongatedbody 45 by suitable means such as brazing at location 54 which is thesame location at which the shaping ribbon 50 is secured to the distalportion of the elongated body. Preferably, the distal portion 53 is freeto rotate within the twisted seal of the distal end of the balloon 44and means are provided to seal the distal portion 53 therein to allowair to be vented therethrough but not inflation fluid such as shown inU.S. Pat. No. 4,793,350 (Mar et al.). The proximal end of the catheter40 is provided with a multiple arm adapter 55 which has one arm 56 fordirecting inflation fluid through the inner lumen 43 and the interior ofthe balloon 44. The proximal end of the elongated body 45 extendsthrough the adapter 55 and is secured to the torquing handle 57 whichrotates the guiding member 41 within the catheter 40 as the catheter isadvanced through a patient's vascular system. The tubular member 42 maybe formed of suitable plastic material such as polyethylene or polyamideor metals such as stainless steel or NITINOL. All or at least the distalportion of the tubular member 42 may be formed of the superelastic NiTitype alloy material of the invention. In an alternative construction,the elongated body 45 has a flattened distal portion which is secured tothe rounded plug 51.

The elongated body 11 of the guidewire 10 and elongated body 45 of thefixed-wire catheter 40 are generally about 150 to about 200 cm(typically about 175 cm) in length with an outer diameter of about 0.01to 0.018 inch (0.25-0.46 mm) for coronary use. Larger diameter guidewireand guiding members may be employed in peripheral arteries. The lengthsof the smaller diameter sections 14, 15 and 16 can range from about 5 toabout 30 cm. The tapered sections 17, 18 and 19 generally are about 3 cmin length, but these too can have various lengths depending upon thestiffness or flexibility desired in the final product. The helical coils22 and 46 are about 20 to about 40 cm in length, have an outer diameterabout the same size as the diameter of the elongated bodies 11 and 45,and are made from wire about 0.002 to 0.003 inch (0.051-0.076 mm) indiameter. The last or most distal 1.5 to about 4 cm of the coil islongitudinally expanded and preferably made of platinum or otherradiopaque material to facilitate the fluoroscopic observation thereofwhen the guidewire or fixed wire catheter is inserted into a patient.The remaining portion of the coils 22 and 45 may be stainless steel. Thetransverse cross-section of the elongated bodies 11 and 45 is generallycircular. However, the shaping ribbons 24 and 50 and the flatteneddistal section 29 have rectangular transverse cross-sections whichusually have dimensions of about 0.001 by 0.003 inch (0.025-0.076 mm).

The superelastic guiding member of the invention, whether it is theentire elongated body 11 or 45 or just a portion thereof, is preferablymade of an alloy material consisting essentially of about 30 to about52% titanium, about 38 to 50% nickel and the balance one or moreadditional alloying elements in amounts of up to about 20%, preferablyup to about 12% in the case of copper and up to 10% for other additionalalloying elements. The other additional alloying elements may beselected from the group consisting of iron, cobalt, platinum, palladium,zirconium, hafnium and niobium in amounts up to 3% each and vanadium inan amount of up to 10%. The addition of nickel above equiatomic amountswith titanium and the other identified alloying elements increases thestress levels at which the stress-induced austenite-to-martensitetransformation occurs and ensures that the temperature at which themartensite phase transforms to the austenite phase is well below humanbody temperature so that austenite is the only stable phase at bodytemperature. The excess nickel and additional alloying elements alsohelp to provide an expanded strain range at very high stresses when thestress induced transformation of the austenite phase to the martensitephase occurs.

A presently preferred method for making the final configuration of thesuperelastic portion of the guiding member is to cold work, preferablyby drawing, a rod or tubular member having a composition according tothe relative proportions described above to effect a final sizereduction of about 10 to about 75% and then providing a memory impartingheat treatment to the cold worked product at a temperature of about 450°to about 600° C. preferably about 475° to about 550° C., for about 0.5to about 60 minutes. In one preferred embodiment the cold worked productis subjected to tensile stress to hold the product straight during theheat treatment to impart a straight memory thereto. In the embodimentwith a solid wire product, the final cold work preferably ranges fromabout 30 to about 70% and the heat treatment temperature ranges fromabout 450° to about 600° C., preferably about 475° to about 550° C. Inanother presently preferred embodiment with tubular products, the finalcold work ranges from about 10 to about 40% and the final memoryimparting heat treatment temperature ranges are the same as mentionedabove. Typical initial transverse dimensions of the rod or the tubularmember prior to cold working are about 0.045 inch (1.14 mm) and about0.25 inch (6.35 mm) respectively. If the final product is to be atubular product, a small diameter ingot, e.g. 0.20 to about 1.5 inch(5.1-38.1 mm) in diameter and 5 to about 48 inches (12.7-122 cm) inlength, may be formed into a hollow tube by extruding or by machining alongitudinal center hole through a solid rod and grinding the outersurface thereof smooth.

After each drawing step, except the last, the solid rod or tubularmember is preferably annealed at a temperature of about 600° to about800° C., typically about 675° C., for about 15 minutes in air or aprotective atmosphere such as argon to relieve essentially all internalstresses. In this manner all of the specimens start the subsequentthermomechanical processing in essentially the same metallurgicalcondition so that consistent final properties are obtained. Suchtreatment also provides the requisite ductility for effective subsequentcold working. The stress relieved stock is preferably drawn through oneor more dies of appropriate inner diameter with a reduction per pass ofabout 10 to 70%. The final heat treatment fixes the austenite-martensitetransformation temperature. Mechanical straightening prior to the finalheat treatment, particularly when tension is applied during the lastheat treatment ensures a uniform level of residual stresses throughoutthe length of the superelastic material which minimizes or eliminatesguidewire whipping when made of this material when torqued within apatient's blood vessel.

In a typical procedure, starting with solid rod stock 0.100 inch (2.54mm) in diameter, the cold working/heat treatment schedule would beperformed as follows:

1. Cold work 60% from 0.100 inch to 0.0632 inch (1.61 mm) and anneal at675° C. for 15 minutes.

2. Cold work 60% form 0.0632 inch to 0.0399 inch (1.02 mm) and anneal at675° C. for 15 minutes.

3. Cold work 60% by drawing through 2-3 dies from 0.0399 inch to 0.0252inch (0.64 mm) and anneal at 675° C. for 15 minutes.

5. Cold work 69% by drawing through 2-3 dies from 0.0252 inch to 0.014inch (0.36 mm).

6. Mechanically straighten and then continuously heat treat at 510° C.at 1 ft/min (0.0305 m/min) under sufficient tension to impart a straightmemory.

When the cold worked material is subjected to the slightly lower thermaltreatments, it has substantially higher tensile properties and itexhibits stress-induced austenite to martensite phase transformation atvery high levels of stress but the stress during the phasetransformation is not very constant. The addition of mechanicalstraightening prior to the final memory imparting heat treatment undertension will substantially improve the whipping characteristics of thefinal product.

FIG. 4 illustrates an idealized stress-strain relationship of an alloyspecimen having superelastic properties as would be exhibited upontensile testing of the specimen. The line from point A to point Bthereon represents the elastic deformation of the specimen. After pointB the strain or deformation is no longer proportional to the appliedstress and it is in the region between point B and point C that thestress-induced transformation of the austenite phase to the martensitephase begins to occur. There can be an intermediate phase developed,sometimes called the rhombohedral phase, depending upon the compositionof the alloy. At point C the material enters a region of relativelyconstant stress with significant deformation or strain. It is in theregion of point C to point D that the transformation from austenite tomartensite occurs. At point D the stress induced transformation to themartensite phase is substantially complete. Beyond point D thestress-induced martensite phase begins to deform, elastically at first,but, beyond point E, the deformation is plastic or permanent. If plasticdeformation occurs, the strain will not return to zero upon the removalof the stress.

When the stress applied to the superelastic metal is removed, the metalwill recover to its original shape, provided that there was no permanentdeformation to the martensite phase. At point F in the recovery process,the metal begins to transform from the stress-induced, unstablemartensite phase back to the more stable austenite phase. In the regionfrom point G to point H, which is also an essentially constant stressregion, the phase transformation from martensite back to austenite isessentially complete. The line from point I to the starting point Arepresents the elastic recovery of the metal to its original shape.

FIG. 5 illustrates a guidewire 60 embodying features of the inventionwhich comprises an elongated, relatively high strength proximal portion61, a relatively short distal portion 62 which is formed substantiallyof superelastic alloy material and a tubular connector element 63 whichis formed substantially of superelastic alloy material and whichconnects the proximal end of the distal portion 62 to the distal end ofthe proximal portion 61 into a torque transmitting relationship. Thedistal portion 62 has at least one tapered section 64 which becomessmaller in the distal direction. The connector element 63 is a hollowtubular shaped element having an inner lumen extending therein which isadapted to receive the proximal end 65 of the distal portion 62 and thedistal end 66 of the proximal portion 61. The ends 65 and 66 may bepress fit into the connector element 63 or they may be secured thereinby crimping or swaging the connector element or by means such as asuitable adhesive or by welding, brazing or soldering.

A helical coil 67 is disposed about the distal portion 62 and has arounded plug 68 on the distal end thereof. The coil 67 is secured to thedistal portion 62 at proximal location 70 and at intermediate location71 by a suitable solder. A shaping ribbon 72 is secured by its proximalend to the distal portion 62 at the same location 71 by the solder andby the distal end thereof to the rounded plug 68 which is usually formedby soldering or welding the distal end of the coil 67 to the distal tipthe shaping ribbon 72. Preferably, the distal section 74 of the helicalcoil 67 is made of radiopaque metal such as platinum or platinum-nickelalloys to facilitate the observation thereof while it is disposed withina patient's body. The distal section 74 should be stretched about 10 toabout 30%.

The most distal part 75 of the distal portion 62 is flattened into arectangular section and preferably provided with a rounded tip 76, e.g.solder, to prevent the passage of the most distal part through thespacing between the stretched distal section 74 of the helical coil 67.

The exposed portion of the elongated proximal portion 61 should beprovided with a coating 77 of lubricous material such aspolytetrafluoroethylene (sold under the trademark Teflon by du Pont, deNemours & Co.) or other suitable lubricous coatings such as thepolysiloxane coatings disclosed in co-pending application Ser. No.559,373, filed Jul. 24, 1990 which is hereby incorporated by reference.

The elongated proximal portions of the guidewires of the invention forcoronary use are generally about 130 to about 140 cm in length withouter diameters of about 0.006 to 0.018 inch (0.152-0.457 mm). Largerdiameter guidewires may be employed in peripheral arteries and otherbody lumens. The lengths of the smaller diameter and tapered sectionscan range from about 2 to about 20 cm, depending upon the stiffness orflexibility desired in the final product. The helical coils are about 20to about 45 cm in length, have an outer diameter about the same size asthe diameter of the elongated proximal portions, and are made from wireabout 0.002 to 0.003 inch in diameter. The shaping ribbons and theflattened distal sections of the distal portions have rectangulartransverse cross-sections which usually have dimensions of about 0.001by 0.003 inch.

An important feature of the present invention is the capability ofmaintaining the level of the yield stress which effects the stressinduced austenite-to-martensite transformation as high as possible, e.g.above 70 ksi, preferably above 90 ksi for solid products (lower withtubular products), with an extensive region of recoverable strain, e.g.a strain range of at least 4%, preferably at least 5%, which occursduring the phase transformation particularly at relatively constantstress.

FIG. 6 graphically illustrates the relationship of the final austenitetransformation temperature (Af) of a binary NiTi alloy of the inventionwith the temperature to which the alloy is subjected during the heattreatment. The amount of cold work and composition will shift the curveto the other valves but will not change the general shape of the curve.

Because of the extended strain range under stress-induced phasetransformation which is characteristic of the superelastic materialdescribed herein, a guidewire made at least in part of such material canbe readily advanced through tortuous arterial passageways. When theguidewire or guiding member of the invention engages the wall of a bodylumen such as a blood vessel, it will deform and in doing so willtransform the austenite of the superelastic portion to martensite. Uponthe disengagement of the guidewire or guiding member, the stress isreduced or eliminated from within the superelastic portion of theguidewire or guiding member and it recovers to its original shape, i.e.the shape "remembered" which is preferably straight. The straight"memory" in conjunction with little or no nonuniform residual stresseswithin the guidewire or guiding member prevent whipping of the guidewirewhen it is torqued from the proximal end thereof. Moreover, due to thevery high level of stress needed to transform the austenite phase to themartensite phase, there is little chance for permanent deformation ofthe guidewire or the guiding member when it is advanced through apatient's artery.

The present invention provides guiding members for guidewires and fixedwire catheters which have superelastic characteristics to facilitate theadvancing thereof in a body lumen. The guiding members exhibitextensive, recoverable strain resulting from stress induced phasetransformation of austenite to martensite at exceptionally high stresslevels which greatly minimizes the risk of damage to arteries during theadvancement therein.

The superelastic tubular members of the present invention areparticularly attractive for use in a wide variety of intravascularcatheters, such as fixed-wire catheters wherein the Nitinol hypotubinghaving superelastic properties may be employed to direct inflation fluidto the interior of the dilatation balloon. In this case a guiding membermay be secured to the distal end of the superelastic Nitinol tubing andextend through the interior of the inflatable balloon and out the distalend thereof. The guiding member may also be made of the superelasticNitinol of the invention.

Superelastic hypotubing generally has been found to have a slightlylower stress level compared to wire when the austenite is transformed tomartensite. However, this stress level is above 40 ksi and is usuallyabove 70 ksi. The hypotubing of the invention generally may have anouter diameter from about 0.05 inch down to about 0.006 inch (1.27-0.152mm) with wall thicknesses of about 0.001 to about 0.004 inch.(0.0254-0.102 mm) A presently preferred superelastic hypotubing has anouter diameter of about 0.012 inch (0.31 mm) and a wall thickness ofabout 0.002 inch (0.051 mm).

While the above description of the invention is directed to presentlypreferred embodiments, various modifications and improvements can bemade to the invention without departing therefrom.

What is claimed is:
 1. A method of forming an elongated tubular memberwhich is in a thermally stable austenite phase at temperatures less thanabout 40° C., which transforms under stress to a martensite phase andwhich has a straight memory in the austenite phase, said methodcomprising:providing an elongated member formed of an alloy consistingessentially of about 30 to about 52% titanium, about 38 to about 52%nickel and up to a total of about 20% of one or more additional alloyingelements selected from the group consisting of iron, cobalt, chromium,platinum, palladium, copper, vanadium, zirconium, hafnium andniobium:subjecting the elongated member to thermomechanical processingwhich includes a final cold working of about 10 to about 75% andsubsequently a heat treatment at a temperature between about 475° andabout 600° C. while applying tension to the elongated member of up toabout 50% of its room temperature tensile strength.
 2. The method ofclaim 1 further comprising subjecting the elongated member to mechanicalstraightening after the final cold working but before the heattreatment.
 3. The method of claim 1 wherein the heat treatingtemperature is between about 475° and about 550° C.
 4. The method ofclaim 1 wherein the heat treatment is conducted for about 0.5 to about60 minutes.
 5. A method of forming an elongated member which is in athermally stable austenite phase at temperatures less than about 40° C.and which has a straight memory, said method comprising subjecting anelongated member having a composition consisting of a predominant amountof NiTi intermetallic constituent to thermomechanical processing whichincludes a final cold working of about 10 to about 75% and a subsequentheat treatment at a temperature between about 475° and about 600° C.while subjecting the cold worked elongated member to sufficientlongitudinal tension to ensure a straight memory in the member.
 6. Themethod of claim 5 wherein the elongated member is formed of an alloyconsisting essentially of about 30 to about 52% titanium, about 38 toabout 52% nickel and up to a total of about 10% of one or moreadditional alloying elements selected from the group consisting of iron,cobalt, chromium, platinum, palladium, copper, vanadium, zirconium,hafnium and niobium.
 7. The method of claim 6 wherein the heat treatmentis at a temperature from about 475° to about 550° C.
 8. The method ofclaim 6 further comprising mechanically straightening the elongatedmember after the final cold working and before the heat treating.